In medical diagnosis, an X-ray radiograph showing the image of tissues requires both sharp contrast and high resolution to show outlines of structures in the tissue which may be similar in composition to adjacent structures or may be physically small. The exposure of the X-ray film must also be carefully controlled to achieve optimum quality of the exposed film. Image quality in an X-ray film depends upon three factors: the contrast in film blackness between tissue structures of slightly different composition, the sharpness or resolution at edges of different structures, and the average density of exposed film particles. Thicker or denser tissue requires more radiation to achieve properly exposed X-ray film.
There is particular interest in use of X-ray filming for mammography. In most mammographic applications, the optimal exposure time for an X-ray film is 0.6-3.0 seconds. A longer time than 3 seconds might cause excess radiation to the patient, excess heating of the X-ray anode, and blurring of the film due to tissue motion during exposure. A shorter time than about 0.6 second might not offer the sharpest film contrast and highest resolution. Therefore other parameters of the X-ray equipment are selected in order to achieve a proper exposure within this time window. In particular the voltage and current applied to the X-ray anode are selected to optimize the image.
The voltage between cathode and anode of the X-ray tube is optimum when it produces X-ray photons of an energy range such that the particular tissue to be exposed absorbs a sizable number of X-ray photons in its more dense structures (those having higher atomic number) while passing more photons through its less dense structures. A lower peak voltage applied to the X-ray tube produces lower energy X-ray photons which are more easily absorbed by any tissue. For soft tissue the photon energy range must be fairly low to produce a clearly visible difference in the absorption rate of similar structures such as fat, blood vessels, and glandular tissue, none of which absorb photons as readily as bone, calcifications, or cartilage, for example. For a tissue which is fairly dense or fairly thick, the photon energy must be higher in order to avoid having too large a portion of the photons absorbed within the tissue. The goal is to permit an optimum percentage of the X-ray photons to pass through the tissue and into the film. In X-raying bones, the voltage which will distinguish between the bones having an average atomic number of about 13 and surrounding muscle (atomic number about 7.5), fat (atomic number about 6), and other soft tissue (average atomic number about 7.5) is not critical, as these adjacent tissues are quite different in composition. In mammography, however, where the X-ray film must distinguish between fatty tissue having an atomic number of about 6 and glandular tissue having an atomic number of about 7.5, a carefully selected peak voltage is needed in order to take advantage of the difference in absorption rate of these structures having similar composition.
For a given voltage and a given exposure time, the proper exposed film density can be obtained by controlling the X-ray flux (the number of X-ray photons per unit area per unit time). X-ray flux is proportional to current from cathode to anode in the X-ray tube. Maximum current in an X-ray tube depends on the power rating for the tube which in turn depends on the intended exposure time. As exposure time increases, maximum operating power (to avoid overheating the anode and other negative effects) decreases. For a given exposure time the power rating is constant, so that increasing the voltage results in a decrease in maximum current. Maximum current also depends on the area of the anode impinged by electrons and from which X-rays are emitted. The impinged area of the anode as projected in the direction X-rays are emitted is called the focal spot size. The power rating can be increased by increasing the size of the focal spot. However, a larger focal spot size decreases the sharpness of the film image, thus it is desirable to minimize focal spot size and therefore current in order to achieve maximum resolution of the film image. Another option for increasing sharpness when a larger focal spot size must be used and sharpness is also needed is to either locate the object to be radiographed close to the film or to move the source of the X-rays farther from the object. This effect is shown in FIGS. 1a and 1b. Objects 13-a and 13-b are located closer to the film in FIG. 1b than in FIG. 1a and thus show smaller blurred areas 16-a-b and 16-b-b than the blurred areas 16-a-a and 16-b-a shown in FIG. 1a.
Currently, an X-ray technician in preparing to X-ray a particular patient, estimates the density and thickness of the tissue to be penetrated by the X-ray beam and sets the voltage and current (or focal spot size) of the machine to achieve optimum contrast with optimum sharpness. A machine may be controlled manually by a technician who also estimates optimum exposure time and sets the machine for that time. When a more automatic machine turns on it will operate at the set voltage and current until a sensor indicates sufficient film darkening has occurred, at which time the sensor will automatically turn off the machine.
Such a prior art sensor will automatically achieve film darkening which is within an optimum range though film darkening can vary by as much as 30% even when an automatic sensor is used. This use of a sensor to control exposure time is well known and satisfactory for X-raying bones and other tissues in which there is sharp contrast between adjacent structures, however when X-raying soft tissue, the energy of X-ray photons, the flux of emitted photons, and the exposure time must all be accurately controlled in order to get good contrast between the similar structures within the soft tissue.
For a given operating voltage, the exposure time must be adjusted to achieve optimum film blackness. However, if the exposure duration is predicted to be too long, thereby introducing blurring of the film due to tissue motion during the filming, the operating voltage must be increased, thus sufficiently exposing the film in a shorter time. It is desirable to use the lowest possible photon energy and thus the lowest operating voltage in order to achieve the maximum contrast between tissue structures which are similar in their X-ray absorption and thus difficult to distinguish on an X-ray film. Another way to shorten the exposure time, achieving a sufficiently black film without increasing operating voltage, and thus reducing contrast, is to increase the operating current and thus increase the X-ray flux (the number of X-rays per second per unit area being delivered by the machine). Still another way is to use more sensitive film. An increased flux shortens the exposure time needed to achieve a given density of exposed film particles, since density of exposed film particles is directly proportional to the flux multiplied by time. An increased flux is achieved by increasing the size of the focal spot on the X-ray tube anode while also increasing the current of electrons hitting this focal spot. In some X-ray systems it is also possible to shorten the source to tissue distance in order to increase the X-ray flux passing through the tissue. This action has a number of limitations including higher surface exposure dose for the exposed tissue, degradation of resolution, less space to position the patient comfortably, and others.
It is not possible to increase the current to a focal spot of a given size beyond the rated current without melting that spot on the anode, thus to increase current the electrons must be defocused to impinge on a larger anode area, in turn causing X-rays to be emitted from a larger area. This larger area produces a reduction in sharpness of the film image as can be seen in FIGS. 1a and 1b (prior art). FIGS. 1a and 1b depict an X-ray source, collimator, objects to be shown on film, and the film. As shown in FIG. 1a, the size of the focal spot causes a defocusing of the image on the film, generating small penumbrae 16-a-a and 16-b-a. However, as shown in FIG. 1c, when the size of the focal spot is increased in order to increase the flux and decrease the exposure time, the size of this blurred area also increases.
FIGS. 2a, 2b, and 2c show film darkening along lines 2a, 2b, and 2c in FIGS. 1a, 1b, and 1c respectively. The penumbrae generally numbered 16 indicate areas of unsharpness in the image. It is desirable to keep these areas as small as possible. Note in FIG. 2a that the entire small object 13-b is represented by penumbrae 16-b-a and might well be unobserved on film. One way to reduce this blurring is to locate the object to be radiographed close to the film, as shown in FIGS. 1b and 2b, another is to move the X-ray source farther from the object. It is desirable to keep the focal spot size as small as possible to avoid defocusing from the larger focal spot in FIG. 1c.
In order to provide a quantitative understanding of the relationship between film optical density (darkness) and operating voltage, operating current, exposure time, tissue atomic number, tissue thickness and distance from X-ray source to film, the following mathematical explanation is provided.
The energy of photons emitted from the X-ray tube falls within an energy spectrum such as those shown in FIG. 3. The maximum photon energy emitted from an X-ray tube equals the maximum energy of the cathode stream electrons impinging on the X-ray tube anode, which in turn depends on the peak applied voltage between the cathode and anode of the X-ray tube and on the voltage wave form. Emitted photons have an energy spectrum which depends on the anode composition as well as the cathode-to-anode voltage drop. FIG. 3 shows energy distribution of emitted photons for tungsten and molybdenum targets when operated at voltages of 24 kilovolts and 28 kilovolts, respectively.
The X-ray flux, or number of emitted X-ray photons per second per unit area, varies as the square of the cathode-to-anode peak voltage and inversely as the square of the distance from the anode: EQU I=cV.sup.2 /D.sup.2 ( 1)
where
I is X-ray photon flux, PA0 V is X-ray tube peak operating voltage, PA0 D is the distance from the anode to the point where flux is measured, and PA0 c is a proportionality constant. PA0 I.sub.out is flux of X-ray photons after passing through the object, PA0 I.sub.in is flux of X-ray photons before passing through the object, PA0 .mu. is the attenuation coefficient of the object at the particular X-ray energy, and PA0 x is the thickness of the object.
As the X-ray photons pass through an object which attenuates X-ray flux, they are attenuated according to the exponential attenuation law: EQU I.sub.out =I.sub.in e.sup.-.mu.x ( 2)
where
Since attenuation coefficient .mu. is a function of photon energy, the energy distribution of X-rays after passing through an object differs from the incident energy distribution.
Attenuation of the emitted X-rays varies with atomic number of the material through which the X-rays pass as well as with the energy of the X-ray photons. X-ray flux is attenuated as it passes through an object for two reasons: absorption and scatter. Absorption attenuation is directly proportional to the third power of the average tissue atomic number and inversely proportional to the third power of photon energy, which is directly proportional to voltage. Thus, ##EQU1## where .mu..sub.abs is the attenuation coefficient due to absorption,
Z is the atomic number of the tissue, and PA1 V is the operating voltage of the X-ray machine. PA1 g is a proportionality constant. PA1 f is a proportionality constant.
Scatter attenuation (the Compton effect) decreases inversely as operating voltage increases and is largely independent of tissue atomic number. ##EQU2## where .mu..sub.sc is attenuation coefficient due to scatter, and
Thus, the total attenuation coefficient on passing through an object due to both absorption and scatter is ##EQU3## Thus, X-ray flux after passing through an object is EQU I.sub.out =I.sub.in e.sup.-(gZ.spsp.3.sup./V.spsp.4.sup.+f/v)x ( 6)
Darkening rate of the X-ray film is directly proportional to X-ray flux at the particular location on the film. Thus it is clear why the operating voltage of the X-ray tube is important in controlling the contrast of the film, and why contrast between structures of slightly different average atomic number increases as operating voltage decreases.
In the past, it has been known that optimum operating voltage is a function of tissue absorption coefficient or tissue density. X-ray technicians have generally used tables of optimum operating voltage and current as a function of tissue density.
For X-raying dense tissue such as bone (Z.apprxeq.13) the preferred X-ray tube voltage is around 85 kilovolts. For soft tissue (Z.apprxeq.7) the preferred voltage for achieving moderate attenuation within the tissue is 20 to 35 kilovolts. When using a molybdenum target X-ray tube for a mammogram, for an average size breast having an average proportion of glandular tissue and thus an average atomic number of approximately 6.8, an average density of about 0.98, and an average compressed tissue thickness of 4-5 cm, a 28 kilovolt setting gives optimum contrast. For an average size breast having a high proportion of fatty tissue with an average atomic number of 6, a density of approximately 0.9, and an average compressed tissue thickness of 4-5 cm, a 26 kilovolt setting gives optimum contrast. For an average size breast dense in glandular tissue and having an average atomic number of approximately 7.5, a density of about 1.05, and compressed tissue thickness of 4-5 cm, a 30 kilovolt setting gives optimum contrast. For a large breast dense in glandular tissue having a compressed tissue thickness of 6-7 cm, a 32 kilovolt setting gives optimum contrast. When using a tungsten target X-ray tube, voltage settings for an average size breast having fatty, average, dense glandular, and thick dense glandular composition will be 23, 24, 25, and 26 kilovolts, respectively. A lower voltage setting gives higher film contrast but requires longer exposure time and results in more radiation absorption to the tissue.
As can be understood from the thickness factor x in equations 2 and 6, thinner tissue requires less radiation for proper film exposure. Thinner tissue also produces a film where structures are easier to observe because fewer structures are superimposed one above another. In the case of a mammogram, the breast is compressed between parallel plates in order to reduce the thickness which must be penetrated by the X-ray beam. Compression can result in a reduction of X-ray tube operating voltage from 30 to 35 kilovolts for an uncompressed breast to 22 to 24 kilovolts (tungsten X-ray tube) for the breast when compressed. This voltage reduction increases film contrast without producing increased radiation exposure to the patient. Such compression has the additional advantage of providing tissue of uniform thickness over most of the area being radiographed and thus improving quality of the produced film. In addition improved quality occurs because fewer tissue structures are superimposed, and there is less scatter radiation. Therefore breast compression is a widely used technique today.
If the technician has not properly estimated average tissue density, the selected voltage and current may not produce optimum contrast and sharpness. After developing the film, the technician or the radiologist may discover that it is necessary to take another film, using a different voltage or current in order to achieve a good enough quality film for a reliable diagnosis.
A method and device are desired which give good film quality in every film in order to reduce the radiation dosage to the patient, the cost of producing a satisfactory film, the inconvenience and discomfort to the patient, the need for technician expertise and experience, and the need for a radiologist to be present to examine the film, thus giving a high patient throughput and lower fees. In particular, in the field of mammography, there is need for a better method and device to produce mammograms quickly and accurately so that a radiologist can make an accurate diagnosis, while the patient receives the service at low cost and doesn't have to wait to see if the X-ray image was satisfactory or return for another session.